The present invention relates to a charged-particle beam irradiation method and system for performing a medical treatment such as a cancer treatment through irradiation with a charged-particle beam, and more particularly to a charged-particle beam irradiation method and system in which an affected part can be irradiated with a charged-particle beam in conformity of the shape of the affected part.
In the case where a cancer treatment is performed by use of a charged-particle beam such as a proton beam with a high energy generated by an accelerator or the like, it is required that an area having a diameter of about 20 cm should be irradiated with a proton beam having an energy of about 230 MeV at the highest. The conventional method for realizing this has been disclosed by W. T. Chu et al, xe2x80x9cInstrumentation for treatment of cancer using proton and light-ion beamsxe2x80x9d, Review of Science Instrument, Vol. 64, No. 8 (August 1993), pp. 2092-2093. In the disclosed method, an affected part is divided into a plurality of layers in the direction of depth in a body and is scanned layer by layer through irradiation with a charged-particle beam in conformity to the shape of each layer.
FIG. 9 shows the construction of a charged-particle beam irradiation system disclosed by the Chu et al""s article. Referring to FIG. 9, a charged-particle beam 90 ejected from an accelerator is adjusted in energy by a degrader 17 so that the irradiation of a plurality of layers 210 to 212 in an affected part 202 of a body 201 with the adjusted beam is made in a sequence from a deeper layer to a shallower layer. The beam is scanned by use of first and second scanning electromagnets 31a and 31b which are disposed in the irradiation system so that the directions of deflection are orthogonal or vertical and horizontal in the plane of each layer.
The Chu et al""s article has disclosed charged-particle scanning methods including a wobbler scanning method in which a beam is circle-wise scanned, a raster scanning method in which a beam is zigzag-wise scanned, and a pixel scanning method in which a beam is pixel-wise scanned. FIG. 10 shows a charged-particle beam irradiation method based on the raster scanning method. As shown in FIG. 10, a charged-particle beam 220 is zigzag-wise scanned in the first layer 210 in conformity to the shape of the first layer 210. A similar scanning is made in the n-th layer 212.
FIG. 11 shows a dose profile 230 (or a relationship between depth and dose) in the case where the irradiation is made with a charged-particle beam having a high energy and a dose profile 231 in the case where the irradiation is made with a charged-particle beam having a high energy. As shown in FIG. 11, the dose profile of the charged-particle beam has the value 240 or 241 of a dose peak called Bragg peak. A beam penetration depth providing the Bragg peak becomes larger as the energy is higher. It is also shown in FIG. 11 that the irradiation with the charged-particle beam is made with a small dose even at depth portions shallower than the Bragg peak providing portion. Referring to FIG. 10, this shows that when the irradiation with the charged-particle beam 220 is made for the first layer 210, a region 222 of the n-th layer 212 is also subjected to the irradiation with the same charged-particle beam 220. Accordingly, in the case where the irradiation with a charged-particle beam 221 is made for the n-th layer 212, it is required that the dose of a beam portion (indicated by dotted line) for irradiation of the region 222 should be reduced. Though only the first layer and the n-th layer are shown in FIG. 10 for simplification of illustration, the actual irradiation of the n-th layer amounts to the superimposed irradiation for the first to (nxe2x88x921)th layers. Therefore, when the irradiation is to be made for the n-th layer, it is necessary that a dose for the beam portion indicated by dotted line in the n-th layer should be equal to or smaller than, for example, one tenth (at the largest ratio) as compared with a dose for a beam portion indicated by solid line.
For such requirements, the Chu et al""s article has proposed two irradiation methods as follows. In a first method, the scanning speed of a charged-particle beam at the time of irradiation of each layer is constant while the intensity of the charged-particle beam is reduced when the region 222 is irradiated. In a second method, the intensity of a charged-particle beam at the time of irradiation of each layer is constant while the scanning speed of the charged-particle beam is increased when the region 222 is irradiated. With each of the first and second methods, it is possible to reduce the radiation dose of the charged-particle beam in the region 222.
In the first method, however, it is required that while one layer is being irradiated with a beam, the intensity of the beam should be changed greatly in accordance with an irradiation position. Namely, there is a problem that a large change in intensity of each charged-particle beam, for example, from1 to {fraction (1/10)} is needed in the period of 0.1 to 2 seconds when one layer is irradiated, which complicates the control of the accelerator ejecting the beam.
In the second method, it is required that the scanning speed of a beam at the time of irradiation of the region 222 should be increased to, for example, 10 times, which needs a large change in magnetic field intensity of the scanning electromagnet with time. Accordingly, there is a problem that a power supply voltage of the scanning electromagnet becomes high, thereby increasing the cost of a power supply for the scanning electromagnet.
An object of the present invention is to provide a charged-particle beam irradiation method and system in which the control of an accelerator ejecting a charged-particle beam is simplified and the cost of a power supply for a scanning electromagnet can be reduced.
A first invention for attaining the above object is characterized in that in a charged-particle beam irradiation method in which while a charged-particle beam ejected from an accelerator is scanned by an electromagnet, each layer resulting from the division of an affected part into a plurality of layers in the direction of progression of the charged-particle beam is irradiated with the charged-particle beam, wherein the intensity of a charged-particle beam for irradiation of a first layer is made lower than the intensity of a charged-particle beam for irradiation of a second layer existing at a position deeper than the first layer in the beam progressing direction, and a scanning speed in the first layer is changed between a portion of the first layer subjected to irradiation at the time of irradiation of the second layer and a portion of the first layer subjected to no irradiation at the time of irradiation of the second layer.
With the construction of the first invention in which the intensity of the charged-particle beam for irradiation of the first layer is made lower than the intensity of the charged-particle beam for irradiation of the second layer, the scanning speed of the charged-particle beam for irradiation of the first layer can be lowered, thereby making it possible to lower a voltage to be applied to the electromagnet. As a result, it is possible to reduce the cost of a power supply for the electromagnet. Also, with the construction in which the scanning speed is changed between the portion of the first layer subjected to irradiation and the portion of the first layer subjected to no irradiation, it is possible to adjust the accumulative dose amount of a portion subjected to superimposed irradiation. Further, since there is no need to make a large change of the intensity of the charged-particle beam in a short time, the control of the accelerator is simplified.
A second invention for attaining the above object is characterized in that in a charged-particle beam irradiation method in which while a charged-particle beam ejected from an accelerator is scanned by an electromagnet, each layer resulting from the division of an affected part into a plurality of layers in the direction of progression of the charged-particle beam is irradiated with the charged-particle beam, wherein the intensity of a charged-particle beam for irradiation of each layer is made lower as the position of that layer becomes shallower in the beam progressing direction, and a scanning speed in a shallower layer is changed between a portion of the shallower layer subjected to irradiation at the time of irradiation of a deeper layer and a portion of the shallower layer subjected to no irradiation at the time of irradiation of the deeper layer.
With the construction in the second invention in which the intensity of the charged-particle beam for irradiation of each layer is made lower as the position of that layer becomes shallower in the beam progressing direction, it is possible to lower a voltage to be applied to the electromagnet. As a result, it is possible to reduce the cost of a power supply for the electromagnet. Also, with the construction in which the scanning speed is changed between the portion of the shallower layer subjected to irradiation and the portion of the shallower layer subjected to no irradiation, it is possible to adjust the accumulative dose amount of a portion subjected to superimposed irradiation is possible. Further, since there is no need to make a large change of the intensity of the charged-particle beam in a short time, the control of the accelerator is simplified.
A third invention for attaining the above object has the features of the first or second invention and is further characterized in that the accelerator includes a synchrotron for ejecting a charged-particle beam through the application of a high-frequency electric field thereto, and the intensity of the charged-particle beam is controlled by controlling the high-frequency electric field.
With the construction in the third invention in which the intensity of the charged-particle beam is controlled by controlling the high-frequency electric field, it is possible to shorten a time required for the change of the beam intensity, thereby shortening a treatment time.
A fourth invention for attaining the above object has the features of the third invention and is further characterized in that the high-frequency electric field is generated from an electrode applied with a high-frequency electric power, and the high-frequency electric field is controlled by controlling the power value of the high-frequency electric power.
With the construction in the fourth invention in which the high-frequency electric field is controlled by controlling the power value of the high-frequency electric power, the control of the high-frequency electric field is simplified.
A fifth invention for attaining the above object has the features of the first or second invention and is further characterized in that the intensity of the charged-particle beam is controlled by controlling the amount of ions injected into the accelerator.
With the fifth invention, since the amount of ions injected into the accelerator can be suppressed to the minimum required, it is possible to reduce unnecessary beams in the accelerator, thereby reducing the (radio) activation of the equipment.
A sixth invention for attaining the above object has the features of the first or second invention and is further characterized in that the scanning of the charged-particle beam is performed on the basis of the dose value of the charged-particle beam.
With the construction in the sixth invention in which the charged-particle beam is scanned on the basis of the dose value thereof, it is possible to control a dose in each layer accurately even if the intensity of the charged-particle beam ejected from the accelerator has some variations.
A seventh invention for attaining the above object has the features of the first or second invention and is further characterized in that a layer to be irradiated with the charged-particle beam is changed by changing the energy of the charged-particle beam, and the change in energy is made by a degrader disposed on an orbit of the charged-particle beam.
With the construction in the seventh invention in which the energy of the charged-particle beam is changed by the degrader, the control of the accelerator is simplified.
An eighth invention for attaining the above object is characterized in that in a charged-particle beam irradiation system in which while a charged-particle beam ejected from an accelerator is scanned by electro-magnet means, each layer resulting from the division of an affected part into a plurality of layers in the direction of progression of the charged-particle beam is irradiated with the charged-particle beam, the system comprises intensity control means for making the intensity of a charged-particle beam for irradiation of a first layer lower than the intensity of a charged-particle beam for irradiation of a second layer existing at a position deeper than the first layer in the beam progressing direction, and scanning speed changing means for changing a scanning speed in the first layer between a portion of the first layer subjected to irradiation at the time of irradiation of the second layer and a portion of the first layer subjected to no irradiation at the time of irradiation of the second layer.
With the eighth invention, there are obtained effects similar to those in the first invention.
A ninth invention for attaining the above object is characterized in that in a charged-particle beam irradiation system in which while a charged-particle beam ejected from an accelerator is scanned by electromagnet means, each layer resulting from the division of an affected part into a plurality of layers in the direction of progression of the charged-particle beam is irradiated with the charged-particle beam, the system comprises intensity control means for making the intensity of the charged-particle beam for irradiation of each layer lower as the-position of that layer becomes shallower in the beam progressing direction, and scanning speed changing means for changing a scanning speed in a shallower layer between a portion of the shallower layer subjected to irradiation at the time of irradiation of a deeper layer and a portion of the shallower layer subjected to no irradiation at the time of irradiation of the deeper layer.
With the ninth invention, there are obtained effects similar to those in the second invention.
A tenth invention for attaining the above object has the features of the eighth or ninth invention and is further characterized in that the intensity control means is constructed to control the intensity of the charged-particle beam by controlling a high-frequency electric field applied when the charged-particle beam is ejected from the accelerator.
With the tenth invention, there are obtained effects similar to those in the third invention.
An eleventh invention for attaining the above object has the features of the tenth invention and is further characterized in that the intensity control means is constructed to control the high-frequency electric field by controlling the power value of a high-frequency electric power applied to an electrode which generates the high-frequency electric field when the charged-particle beam is ejected from the accelerator.
With the eleventh invention, there are obtained effects similar to those in the fourth invention.
A twelfth invention for attaining the above object has the features of the eighth or ninth invention and is further characterized in that the intensity control means is constructed to control the intensity of the charged-particle beam by controlling the amount of ions injected into the accelerator.
With the twelfth invention, there are obtained effects similar to those in the fifth invention.
A thirteenth invention for attaining the above object has the features of the eighth or ninth invention and is further characterized in that there is provided electromagnet control means for controlling the electromagnet on the basis of the dose value of the charged-particle beam.
With the thirteenth invention, there are obtained effects similar to those in the sixth invention.
A fourteenth invention for attaining the above object has the features of the eighth or ninth invention and is further characterized in that there is provided a degrader disposed on an orbit of the charged-particle beam for making the change in energy of the charged-particle beam.
With the fourteenth invention, there are obtained effects similar to those in the seventh invention.